Police at the scene of a terrorism-related raid in Dewsbury, West Yorkshire, in April.
Photograph: Aaron Chown/PA

The number of terrorism-related arrests in Britain hit a record high after a series of attacks around the country last year, official figures show.

In the year ending 31 March, 441 people were held on suspicion of terrorism-related activity, the highest number of arrests in a year since data collection started in 2001, and an increase of 17% on the 378 in the previous year.

The Home Office said the rise was partly due to a number of arrests made following attacks in London and Manchester last year. The number of terror-related arrests in Britain since the 9/11 attacks in 2001 has passed the 4,000 mark, standing at 4,182 at the end of March.

Assistant commissioner Neil Basu, head of counter-terrorism policing in the UK, said: “With the terrorist attacks of 2017 we saw a genuine step-change in momentum. As a result, our operational activity increased to meet the new and emerging threats we now face.

“A year on and our activity continues to be at unprecedented levels; shown, not least, by the fact that, in the past year, working together with the security services we have stopped an average of one terrorist attack every month. Police, together with the security services, are determined to make the UK as hostile an environment for terrorists as possible.”

Since the Westminster attack in March 2017, there have been 12 Islamist terrorist plots foiled in the UK by police and the intelligence services, with a further four attacks by far-rightwing extremists also stopped in this period.

The number of prisoners jailed for terrorism-related offences is also at the highest level since records began after an increase of nearly a third in one year.

There were 228 persons in custody in Britain as at 31 March, a 27% increase from 180 in the previous year, the Home Office data shows, the highest number in custody for terrorism-related offences since data collection began in April 2009.

The figures also show 48 prisoners held for terrorism-related offences were released from custody in Britain in 2017.

Guardian analysis has revealed police and security services face a surge in the number of convicted terrorists released from prison, prompting warnings over how to manage them. The security minister, Ben Wallace, said the findings were a concern.

Sajid Javid, the home secretary, unveiled an updated counter-terrorism strategy last week, which included revised provisions for managing terrorist offenders in the community.